Collaborative Research: A global examination of the subduction zone flow field from seismic anisotropy

当一个构造板块俯冲回地球时会发生什么？在俯冲带的内部？令人惊讶的是，这个简单的问题仍然是地球科学中未解决的基本问题之一。下行板块（通常称为板块）周围的流动模式通常是用地震各向异性来研究的，地震各向异性是地幔的一种特性，可以通过记录地震波来检测。特别是，横波分裂的测量可以用来表征俯冲带的地震各向异性。从这些测量结果中越来越清楚地看出，最简单的二维板上角流和板下夹带流模型可能是不正确的。然而，人们尚未就另一种模式达成共识。在这个项目中，我们将进行全球横波分裂观测的调查，目的是了解是什么控制了俯冲带区域的地幔流场。为了做到这一点，我们将确定描述俯冲带动力学的参数，这些参数似乎对横波分裂施加了一级控制。初步的工作已经确定了与海沟迁移速度大小有关的板块上下各向异性的系统变化。这就产生了一个假设，即三维流动在板块下方占主导地位，并与地幔楔中的二维角流相互作用。除了对俯冲带地震各向异性进行系统评估外，我们还将构建板块上下地幔流动的实验室和数值模型，以确定剪切波分裂测量中流场的诊断特征，并探索我们的模型对地幔动力学的影响。这个项目是一个跨学科的努力，利用地震各向异性观测和地球动力学建模来理解和描述伴随俯冲的地幔流场的特征。随着横波分裂作为一种测绘地幔流动的工具的日益普及，现在可以获得来自俯冲带的大量数据。因此，有必要在全球范围内开展分裂观测调查，以了解哪些俯冲参数（如收敛速度、海沟迁移和曲率、俯冲板块的年龄和扩张历史、板块倾角和形态、地震活动性、弧长、上覆板块厚度和应力以及火山活动）控制着俯冲带流场。根据初步调查，我们假设在板块下以三维流动为主，并且这种流场与地幔楔中的二维角流相互作用。我们将用实验室和数值模拟研究来补充我们的主要观测地震学目标。这项正演模拟工作将用于验证我们的工作模型的预测，制定替代假设，确定流场的任何二阶效应，并探索我们的工作模型对更大规模地幔动力学的影响。全球许多地区对各向异性的限制以及地震观测、实验室和数值模拟的结合表明，解决下行板块与周围地幔相互作用的基本问题是触手可及的。

What happens when a tectonic plate plunges back into the Earth?s interior at a subduction zone? Surprisingly, this simple question remains one of the fundamental unsolved problems in earth science. The flow pattern around the downgoing plate (often called the slab) is typically studied using seismic anisotropy, a property of the mantle that can be detected by recording seismic waves. In particular, measurements of shear wave splitting can be used to characterize seismic anisotropy in subduction zones. It is increasingly clear from such measurements that the simplest model of two-dimensional corner flow above the slab and entrained flow beneath the slab is likely incorrect. However, consensus on an alternative model has not been forthcoming. In this project, we will undertake a global survey of shear wave splitting observations with the goal of understanding what controls the mantle flow field in subduction zone regions. To do this, we will identify parameters that describe subduction zone dynamics that appear to exert a first-order control on shear wave splitting. Preliminary work has identified systematic variations in anisotropy both above and below the slab linked with the magnitude of trench migration velocity. This has led to the hypothesis that 3-D flow dominates beneath the slab and interacts with 2-D corner flow in the mantle wedge. In addition to a systematic evaluation of seismic anisotropy in subduction zones, we will construct laboratory and numerical models of mantle flow above and below the slab to identify diagnostic features of the flow field in shear wave splitting measurements and to explore the implications of our model for mantle dynamics. This project constitutes an interdisciplinary effort to understand and characterize the character of the mantle flow field that accompanies subduction using observations of seismic anisotropy and geodynamical modeling. With the increasing popularity of shear wave splitting as a tool for mapping mantle flow, a copious amount of data from subduction zones is now available. It is timely, therefore, to undertake a global survey of splitting observations with the goal of understanding which subduction parameters (such as convergence velocity, trench migration and curvature, age and spreading history of the downgoing plate, slab dip and morphology, seismicity, arc length, overriding plate thickness and stress, and volcanic production) appear to control the subduction zone flow field. From a preliminary survey, we hypothesize that 3-D flow dominates beneath the slab and that this flow field interacts with 2-D corner flow in the mantle wedge. We will complement our primary observational seismology goals with laboratory and numerical modeling studies. This forward modeling work will be used to validate the predictions of our working model, formulate alternative hypotheses, identify any second-order effects on the flow field, and explore the implications of our working model for larger-scale mantle dynamics. The availability of constraints on anisotropy from many regions around the globe and the combination of seismological observations and laboratory and numerical modeling suggest that a solution to the fundamental problem of interaction between downgoing slabs and the surrounding mantle is within reach.